A wide variety of methods and apparatus have been designed for the conversion of the energy in sea waves.
Relevant prior inventions include patent documents U.S. Pat. No. 4,077,213, U.S. Pat. No. 4,210,821, U.S. Pat. No. 4,118,932, U.S. Pat. No. 4,686,377, GB1448204 and U.S. Pat. No. 4,684,815. These patents all describe mechanisms for the capture of wave energy based on an arrangement of rafts or floats, either fixed to a permanent base or a moored structure.
These documents, and others, suggest the use of hydraulic take-off systems based on sea water as the working fluid. Three patents suggest the use of single degree of freedom joints and all share a configuration based on a chain of articulated floats with the axis of rotation aligned parallel to each other and perpendicular to the incoming wave direction
More recently WO 00/17519 describes an apparatus composed of a long chain of predominantly cylindrical members capable of controlled motion in 2 degrees of freedom.
The complexity and high capital expenditure associated with developing an offshore wave energy converter has limited the progress of the vast majority of devices.
Travelling ocean waves are a mechanism of energy transmission. The frictional interaction of wind at the air water interface, or sea surface, with the slower moving, and considerably denser, water particles results in a transfer of energy and the creation of ocean waves.
Beneath the sea surface, the energy flux manifests itself as three dimensional pressure and velocity fields. The transmission of energy occurs through the elastic vibration of these fields, with the pressure component manifesting itself as an elevation of the local surface, and therefore potential energy, while the complementary velocity field manifests itself as kinetic energy. In this way, energy is continually transformed between these two types of energy, the transformation being so conservative that a deep water wave can travel several thousand kilometers and lose only a few percent of the energy it started with. The energy in the waves is not restricted to the free surface, nor is it present in the full extent of the water column; rather it decreases exponentially.
It is convenient to conceptualise the wave regime as a regular wave, but in reality a real sea state will be composed of a superposition of many waves. This results in a spectrum where the energy, or quantity, of waves of a particular frequency can be represented by a single equation.
Furthermore a real sea state comprises not only of waves of varying frequency but also of varying direction. This situation tends to depend on the geography of the deployment site. In locations where a long fetch is present such as the west coast of the European continent or the south coast of New Zealand, travelling waves will have more time, and encouragement, to synchronise their periods and direction of travel. In this situation waves will exhibit less directional spreading and will tend to be larger with a more defined peak in the spectrum. By contrast, regions where more than one possible fetch exists—such as the North Sea or the north-west coast of New Zealand—or where reflection or shorter fetches are present waves will tend to show more spreading in both their frequency and direction.
The energy transmitted by a travelling wave has a dependence upon the square of the wave height. Revenue from a wave energy conversion plant is directly dependant upon the power produced which is clearly dependant on the energy available to the absorber. A good wave energy site is conventionally defined as one having a mean power density of between 20 and 70 kW/m. It should be noted that while many sites with these densities exist, there are many more that fall beneath this threshold.
Historically, the development of wave energy converters has been an inherently risky undertaking. Of the devices deployed to date, 80% or more have failed—a significant number ending up on the seabed or washed up on the beach.
A further difficulty with developing wave energy devices for the more aggressive offshore sites is that the efficiency of capture, or absorption, of the energy in the waves depends to some extent on the size of the device. To achieve good absorption, a small device will require an amplitude of motion considerably larger than the amplitude of the incident waves. Through complex control this is possible to a certain extent, but it is difficult, and more so as additional degrees of freedom are included.
By contrast, however, a small device has many operational advantages over larger devices. A small device will be less likely to suffer from internal stresses due to variations in pressure loading across the structure, and will therefore gain through reduced material costs. It will also be considerably cheaper to install.
Experience with wave energy deployments to date has shown that the cost of marine operations, particularly mooring operations, accounts for a significant proportion of the overall project cost (typically ˜50%). An economically viable marine energy device will therefore require an efficient mooring and operations procedure.
Furthermore, the mooring and deployment operation is not always a ‘once in a lifetime’ event for many current designs. Wave energy devices tend to be complicated dynamic machines with multi-component power take off systems and many unavoidable failure mechanisms. A few devices such as WaveDragon and the Orecon multi-chamber resonant OWC are being designed so that general maintenance operations can be carried out on-site, however, they pay for this advantage with increased size and one off mooring costs.
The majority of designs currently under development fall under the category of single point absorbers, and a majority of these aim to capture energy from a single degree of freedom—usually heave. There are, however, other approaches that have either been proposed or are under active research. Perhaps the most famous of these being Salter's Duck.
Generally speaking a wave energy PTO (Power Take Off) system will aim to achieve the following:
1) Rectification of the energy flow—to allow power capture both on the out and return strokes.
2) Step up in speed, to allow smaller secondary electrical conversion machinery
3) Power smoothing to allow generators to operate closer to optimum efficiency and for grid compliance.
Many PTO systems have been proposed for wave energy conversion. Each of them has their advantages and disadvantages. Some existing approached are summarised in the following:
Hydraulic systems have long been considered suitable for the primary PTO conversion stage.
However they require a sophisticated system with many auxiliary systems such as cooling, redundancy, and control. High pressure hoses are a notorious cause of failure especially when subjected to pulsating loads and when the hoses are required to pass fluid across moving joints. The latter difficulty can be alleviated by using rotating unions in the case when the joint has one, rotating, degree of freedom.
The control system of a marine energy device is required to perform several tasks. These include optimising the performance and protecting the system. From the point of view of performance, this typically implies a control over the load seen by the primary conversion device such that the hydrodynamic efficiency of the absorber is maximised.
Power is the product of force and velocity, therefore the foremost requirement for hydrodynamic efficiency is that the force and velocity vectors are aligned. The condition when this happens throughout a wave cycle is referred to as resonance. Essentially the absorber should be moving away from the applied force at all times. If this does not happen and the velocity and force vectors are in opposing directions, the kinetic energy of the absorber will be transferred to the fluid and power will be lost from the system. Several control strategies have been proposed and developed for wave energy devices.
The simplest type of performance control is not to have control at all. In this instance a constant damping (proportional to velocity) is applied to the primary conversion device throughout the wave cycle.
One particular problem specific to wave power converters is the so called end stop problem. Physical limitations exist for linear conversion drives such as hydraulic rams or direct drive linear generators that restrict the stroke, or range of motion, of these primary conversion units. There is a certain probability that a given wave energy converter, installed in a specific site, will encounter waves above a certain amplitude. It is not generally economically efficient to design the system to accommodate the largest wave that the device may experience; therefore a situation will occur where a system equipped with a driven linear drive will be required to restrict the response of the system to protect the end stops of the primary power take off component.
Many of the difficulties in designing an economically viable marine energy converter stem from the unpredictability of the marine environment. Structures must be designed and built to withstand the highest waves which may occur only very rarely yet exert disproportionately high forces yet, at the same time; they must also be built to generating efficiently in the smaller waves which are much more common. All power take off components need to be rated to the highest powers, whether it be hydraulic, mechanical or electrical, that they are likely to experience during their lifetime. The device must be able to protect itself in the event that it is subject to incident powers above this rating and this may require the offloading of power take off systems and the dissipation of excessive power.
It is an object of the present invention to address at least some of the issues identified above.